One of the principal uses of corrugated thin metal strip (0.001" to 0.010") has been the production of supports for various kinds of catalysts for carrying out various chemical reactions, such as oxidation, reduction, particulate trapping, etc., and, more recently, in the preparation of catalytic elements for use in catalytic converters positioned in the exhaust lines of internal combustion engines, for example, automotive vehicles, to eliminate by catalytic means various pollutant materials.
Up to present time, supports for the catalyst or catalysts and particulate traps which effect the desired pollution reducing effects have been made from ceramic materials in the form of the familiar "honeycomb". The inner walls of the monolithic honeycombs are coated with a precious metal catalyst or a plurality of catalysts, such as platinum, paladium and/or rodium. Hot exhaust gas coming into contact with the surfaces carrying the catalyst material undergoes chemical change to harmless materials. An early embodiment of a metallic catalyst carrier is described in U.S. Pat. No. 1,636,685 dated July 26, 1927 and issued to Downs. According to Downs, iron particles are treated by dipping into melted aluminum or by milling with powdered aluminum. In a process called calorizing, there takes place an alloying action between the aluminum and the iron, the iron/aluminum surface provides a very satisfactory surface upon which to deposit catalytic materials, e.g., oxide catalysts such as metal oxides of groups V and VI of the periodic tables. These structures are adapted for vapor phase catalytic oxidation of organic compounds. Numerous other patents have issued including the patent to Sutter U.S. Pat. No. 2,658,752 dated Nov. 10, 1953. This patent discloses stainless steel as the base metal for the catalyst. The base metal may be in the form of a wire or screen or other physical form.
The U.S. Patent to Retallick U.S. Pat. No. 4,301,039 dated Nov. 17, 1981 discloses a method of making a metal catalyst support in spirally wound form whereby indentations in the surface will prevent nesting together when the strip is wound as a spiral.
U.S. Pat. No. 4,402,871 to Retallick dated Sept. 6, 1983 discloses a honeycomb catalyst support formed by folding a single layer of metal back and forth upon itself. Each layer in the honeycomb has indentations of uniform height so that the spacing between the layers is equal to this height. A different pattern of indentations is used on alternate layers and the indentations are of opposite sides of the strip in alternate layers. This structure prevents nesting of confronting layers. The more recent structures are made of thin ferritic stainless steel strip of the type referred to by Kilbane in patent application Ser. No. 741,282 filed June 4, 1985, now U.S. Pat. No. 4,686,155, and by Retallic in U.S. Pat. 4,601,999 dated July 22, 1986 which strip is corrugated and then fan folded or folded back and forth upon itself. The surface of the strip is provided with a catalytically active agent for decontaminating an exhaust gas e.g., the exhaust gas generated by an internal combustion engine.
More recent developments have resulted in the production of a strip of metal adapted to receive an alumina wash coat on which is deposited various catalytic material. This strip is corrugated and in order to prevent nesting of the strip when it is folded back and forth upon itself in a zig-zag or accordion folded manner, the corrugations instead of being peaks and valleys lying along straight lines extended perpendicular to the longitudinal marginal edges of the strip, are provided with a discontinuous configuration such as a plurality of chevron or V-shaped structures. Several such V-shaped displacements causing deviations from a normal straight line have been provided. Also, there has been considered a sinusoidal pattern for the peaks and valleys forming the corrugations.
The gears by which these corrugations are formed have been a series of relatively short helical gears carefully mounted on a shaft and matched so that the indentation imprinted in the surface of the thin metal strip is a V-shaped chevron.
These corrugated metal strips have the principal advantage of being nonnesting when folded back and forth upon themselves in a zig-zag manner.
It has been found, however, that the chevron or V-shaped pattern, especially where there are a plurality of such V-shaped projections across the width of the metal strip have quite unexpectedly shown an unusual type of corrosion failure. After prolonged exposure to high temperature and high hydrocarbon content, corrosion is found to occur at the apices of every other chevron. It was then discovered that those apices which were in compression resisted corrosion, while those apices that were in tension were subject to corrosion. It was further learned that those apices which went through the corrugating gears first were in compression while those which trailed and went through last were found to be in tension and subject to corrosion.
This discovery of the effect of the direction of movement of the apices of chevron shaped projections has been determined to be of great utility in an entirely different aspect. That aspect is this:
It has not been found possible to roll a straight corrugations into a ferritic stainless steel thin metal strip where the peaks and valleys are disposed across the entire width of the strip at an angle to the longitudinal marginal edges of the metal strip. The reason for this is that with a helical gear, in attempting to form such corrugations, the metal strip travels laterally and eventually bunches up and jams the entire apparatus. It has been found that by rolling the metal strip in a manner such that the apices of a single chevron type corrugation are in tension, rather than in compression, and the included angle between the sides of the V-shaped chevron is less than about 166.degree., the metal strip will split or can be split easily along the line defined by the apices of the chevron into two separate strips each of which has straight corrugations disposed at an angle between the marginal edges of the strip, and avoiding entirely the problem of lateral travelling of the metal strip and bunching and jamming of the rolling gears as described above.
Now, when the corrugated metal strip formed in the manner stated above is folded back and forth upon itself in a zig-zag manner or accordion folded, the resulting bundle is free of any tendency toward nesting. It is much less expensive to manufacture helical gears to insert a single chevron of the type herein contemplated than it is to form a corrugated surface having many chevrons and therefore many apices alternatingly under compression and tension as for example illustrated in the application of Richard C. Cornelison et al, Ser. No. 830,698 filed Feb. 18, 1986 now U.S. Pat. No. 4,711,099. FIGS. 3 and 4 of said Ser. No. 830,698 show helical gears used to corrugate a thin metal strip with a plurality of V-shaped chevrons across the width of the thin metal strip. These helical gears are extremely expensive to manufacture and require a great deal of patience and skill to get properly aligned on the gear supporting shaft. In the present case, there is a pair of oppositely directed confronting helical gears mounted on the same shaft as opposed to a larger plurality of such gears mounted on the shaft. There is, therefore, a great saving in cost of the corrugating apparatus.